General exhaust gas processing systems are shown in FIG. 15, FIG. 10, and FIG. 11. In the exhaust gas processing system shown in FIG. 15, the exhaust gas containing a large amount of dust emitted from a combustion device such as a boiler 1 using coal as fuel is introduced to a denitration device 2 in which nitrogen oxide contained in the exhaust gas is removed. Then in an air preheater 3, the exhaust gas is heat-exchanged with air for combustion which is supplied to the boiler 1. After most of the dust in the exhaust gas is removed in a dust collector 5 (including a bag filter and an electric static precipitator in the present specification), the exhaust gas is pressurized up by an induced draft fan 6. Sequently, the exhaust gas is introduced to a gas-gas heater (GGH) heat recovery device 4 in which heat is recovered, and then introduced to a wet-type desulfurization device 7 in which sulfur oxide (SOx) contained in the exhaust gas is removed by gas-liquid contacted with the absorber containing a desulfuring agent. The exhaust gas cooled down to the saturated gas temperature in the wet-type desulfurization device 7 is pressurized up by the desulfuring fan 9, heated by a GGH re-heater 8, and emitted from a smokestack 10. Between the GGH heat recovery device 4 and the GGH re-heater 8 are provided interconnecting lines 13 in which a heat medium circulates.
The other exhaust gas processing systems are shown in FIG. 10 and FIG. 11, and their GGH (gas-gas heater) systems are shown in FIGS. 12 and 13. In these drawings, the same components are referred to with the same reference numbers.
In FIG. 10, the exhaust gas from the boiler 1 is flown through an exhaust gas duct 30, introduced to the denitration device 2 in which nitrogen oxide in the exhaust gas is removed, and in the air preheater 3, is heat-exchanged with air for combustion to be supplied to the boiler 1. Next, the exhaust gas is introduced to the GGH heat recovery device 4 in which the exhaust gas is heat-exchanged with the heat medium flowing through the heat recovery device 4, thereby decreasing the temperature of the exhaust gas and also decreasing the electric resistance value of the dust in the exhaust gas. In this condition, the exhaust gas is introduced to an electric static precipitator 5 in which most of the dust in the exhaust gas is removed. Sequently, the exhaust gas is pressurized up by the induced draft fan 6, introduced to the wet-type exhaust gas desulfurization device 7, and subjected to gas-liquid contact with a desulfuring agent-containing liquid so as to remove SOx and part of the dust in the exhaust gas. The exhaust gas cooled down to the saturated gas temperature in the wet-type desulfurization device 7 is heated by the GGH re-heater 8 by a heat exchange with the heat medium supplied from the heat recovery device 4, pressurized up by the desulfuring fan 9, and emitted from the smokestack 10.
FIG. 11 shows a system where there is a wet-type dust collector 19 added between the wet-type exhaust gas desulfurization device 7 and the GGH re-heater 8 in the exhaust gas duct 30 in order to further reduce the dust contained in the exhaust gas at the outlet of the wet-type exhaust gas desulfurization device 7.
In the exhaust gas processing systems shown in FIG. 10 and FIG. 11, the duct collector 5 is installed at a side down stream of the GGH heat recovery device 4 in the exhaust gas duct 30, which results in a decrease in the temperature of the processing gas in the dust collector 5, thereby decreasing the electric resistance of the dust and increasing the efficiency of removing the dust. Thus, it has a high dust removing performance, compared with the exhaust gas processing system shown in FIG. 15.
Since dust emission controls are becoming stricter recently, the exhaust gas processing systems shown in FIG. 10 and FIG. 11 have become main stream processing systems for exhaust gas which contains a large amount of dust emitted from a boiler or the like using coal as fuel.
Next, the GGH systems of the exhaust gas processing systems shown in FIG. 10 and FIG. 11 will be described with reference to FIG. 12 and FIG. 13.
The heat transfer tubes 11 in the GGH heat recovery device 4 and the heat transfer tubes 12-2 in the GGH re-heater 8 are connected with each other via the interconnecting lines 13 where the heat medium is circulated by a heat medium circulation pump 14. In the heat medium circulation system, there is a heat medium tank 15 installed for absorbing the expansion of the heat medium in the system, and there is also a heat medium heater 16 for controlling the temperature of the heat medium so as to keep the operation of the boiler or the like stable. The steam drain generated in the heat medium heater 16 is recovered by a heat medium heater drain tank 17, and then transferred to a boiler-side tank (not illustrated).
The heat transfer tubes 11 of the GGH heat recovery device and the heat transfer tubes 12-2 of the GGH re-heater are generally composed of fin-equipped heat transfer tubes in order to improve the efficiency of heat exchange. Furthermore, on the stage preceding the GGH re-heater 8 is provided a bare tube 12-1 consisting of at least three stages of bare heat transfer tubes with no fins in order to remove (evaporate) corrosive mist scattering from the wet-type exhaust gas desulfurization device 7.
Such a structure is disclosed in Japanese Published Unexamined Patent Application No. 2000-161647 in which the heat medium circulating through the GGH heat recovery device 4 and the re-heater 8 is flown into the bare tube 12-1 so as to increase the surface temperature of the bare tube, thereby removing the scattering mist.
FIG. 13 shows a system configuration where there is a SGH (steam gas heater) 20 installed as the heat transfer tubes composed of the bare tube installed in the stage preceding the fin-equipped heat transfer tubes 12-2 of the GGH re-heater 8 in the system shown in FIG. 12, and steam is introduced to the SGH 20 from outside. The steam drain generating in the SGH 20 is recovered by a SGH drain tank 18 and then transferred to a boiler-side tank (not illustrated).
FIG. 14 shows a simplified side view (FIG. 14(a)) and a cross sectional view taken along the line A-A (FIG. 14(b)) in the case where soot blowers 21 are installed as dust removers for the GGH.
The soot blowers 21 used in the GGH are generally kept inside the exhaust gas duct 30 because the exhaust gas temperature in the GGH is low (160° C. or lower). When the soot blowers 21, which are supplied with steam or air, are in operation, the tubes inserted in the soot blowers 21 go back and forth while rotating (moving vertically in the case shown in FIG. 14), and during movement, steam or air is jetted from the holes formed in the tubes, thereby removing dust and the like accumulated in the heat transfer tubes (fin-equipped heat transfer tubes) 11 and 12-2 of the GGH.
In general, in a heat exchanger with GGH heat transfer tubes (fin-equipped heat transfer tubes), the heat transfer performance of the heat exchanger can be improved by increasing the flow rate of the gas which passes through the heat transfer tube region, thereby reducing the total heat transfer area.
Diminishing the fin pitch of the fin-equipped heat transfer tubes used as the heat transfer tubes (in general, the fin pitch is not more than 5.08 mm) can increase the heat transfer area per heat transfer tube, so as to reduce the number of heat transfer tubes installed in the whole heat exchanger, thereby reducing the size of the heat exchanger.
However, in the aforementioned exhaust gas processing system provided with the GGH, the exhaust gas introduced to the GGH heat recovery device 4 installed at a side down stream of the air pre-heater 3 (a side upper stream of the dust collector 5) in the exhaust gas duct 30 contains a large amount of dust (10 to 50 g/m3N or so). This causes a problem of abrasion (due to ash erosion) over time with the heat transfer tubes 11 of the GGH heat recovery device 4 and their fins, and also the problem of clogging of the regions between the adjacent fins as a result that the dust and SO3 contained in the exhaust gas adhere to the heat transfer tubes 11.
In the GGH re-heater 8 installed at a side down stream of the wet-type exhaust gas desulfurization device 7 in the exhaust gas duct 30, the dust collector 5 and the wet-type exhaust gas desulfurization device 7 remove dust, so its amount is reduced to approximately 20 mg/m3N or lower. Consequently, in the GGH re-heater 8, the abrasion (ash erosion) environment due to the dust is mitigated. However, there are still other problems as follows. The sulfur oxide absorber containing plaster slurry and the like and mist containing a corrosive ingredient which scatters from the devices (the wet-type exhaust gas desulfurization device 7 and the wet-type dust collector 19) installed at a side down stream of the GGH re-heater 8 collide with the fin-equipped heat transfer tubes 12-2 of the GGH re-heater 8, thereby corroding the fin-equipped heat transfer tube 12-2. In addition, the dust adhered to the fins over time clogs the regions between the adjacent fins and between the adjacent heat transfer tubes where the gas flows.
In general, the soot blowers 21 or the like are installed as GGH dust removers, and for effective dust removal of the heat transfer tubes composing the GGH, it is necessary to take some measures, such as increasing the number of soot blowers 21 or increasing the frequency of activating the soot blowers 21.
Normally, the soot blowers 21 are activated (timer control) at a frequency of 3 to 5 times a day. Since the operation of the soot blowers 21 is controlled at a frequency of activation based on the worst conditions assumed in consideration of the problems that the dust adheres to the fins of the fin-equipped heat transfer tubes and clogs the regions between the adjacent fins and between the adjacent heat transfer tubes where the gas flows, an excessive amount of steam tends to be introduced to the duct 30.
Therefore, the object of the present invention is to provide an exhaust gas processing device provided with heat transfer tubes for the GGH heat recovery device and the GGH re-heater, which are structured to solve the aforementioned problems in consideration of the environment with a large amount of dust where the GGH is installed.